Every A/B test is this, to some degree. Every ad run with variants is also. Facebook did specifically do a test to see if emotions spread between people throug

Every A/B test is this, to some degree. Every ad run with variants is also. Facebook did specifically do a test to see if emotions spread between people through the news feed. They didn't create posts to manipulate emotions, but they did track people after seeing emotionally charged posts in their feeds. https://www.pnas.org/content/111/24/8788 https://www.pnas.org/content/111/24/8788